1. Field of the Invention
Embodiments of this invention relate to oil field services operations. Specifically, embodiments of this invention relate to ways to control additives to a hydraulic fracturing operation.
2. Description of the Related Art
Hydraulic fracturing is a process for stimulating oil and gas wells by pumping gel-sand slurries at high pressure into producing rock layers. Once the rock is cracked, the resulting fracture is propped open by the sand carried by the slurry. This fracture serves as a highly conductive path for the oil or gas, and therefore increases the effective well-bore radius. Fluid viscosity is vital for effective proppant placement during fracturing operations. Polysaccharides such as guar and guar derivatives have historically served as the most common viscosity enhancers. They are often crosslinked using borates or metallic crosslinkers such as zirconium and titanium to generate even higher viscosity. Multiple additives are added to each formulation. Pre-job Quality Assurance/Quality Control (QA/QC) is performed on location minutes before beginning to pump to ensure the fluid performs as required.
A major challenge in hydraulic fracturing operations is how to ensure that the fluid that is being pumped continuously is an exact match to the performance it was designed for. Fluid formulations during the treatment are controlled by maintaining given additive concentrations in control through close loop control strategies managed with pumps and flowmeters, for which one point calibration verification is carried out through a pre-job “bucket check”. No redundancy is typically incorporated. Samples of the fluid are manually taken at significant events, (begin of pumping, begin of proppant pumping, change of proppant concentration) but this can only be done sparsely. Typically, visual inspection of the fluid's ability to transport proppant is carried out.
Also, cementing is a process for zonal isolation. In this process, multiple additives, retarders, accelerators, dispersants, foamers are added to the mix water prior to the addition of the cement slurry. Controlling the exact concentration of each of the additives either on the fly or when water is batch mixed is key for the successful execution of the treatment.
Gravel pack, fracturing and packing, matrix acidizing, wellbore clean out, wellbore remediation, conformance control, additive squeeze treatments such as organic and inorganic scale removal treatments, hydrate or asphaltene prevention treatments, well abandonment pills, filter cake removal treatments, and others are all well service operations that require some level of chemical formulation mixing and preparation, and for which the ability to formulate the fluid as per design is very important for the treatment effectiveness, and eventually to be able to respond in real time to predesigned formulation changes, or unforeseen changes required as a result of the reception and or evaluation of stimuli and responses from the formation, the reservoir, or the downhole completion.
For example, FIG. 1 describes the common control of chemical composition in well service treatments when an open loop strategy is used as a method for fluid delivery control. In the figure, a chemical A (110), in a fluid form, such as to deliver a concentration of chemical per unit volume CA0, is pumped through a metering pump (120) at a flow rate FA. The flow rate is typically set by a frequency, voltage, or current proportional to the pumping rate of the pump, as typically determined by calibration, and in multiple occasions might be verified by a mass flowmeter (130). Frequently the calibration is set by the pump manufacturer, given a certain set of physical parameters in the pump (range, size of stroke, etc), and verified prior to the job execution by a volumetric determination of fluid delivered in a pre-set amount of time, what is commonly called a “bucket check.” The actual concentration of chemical A delivered through stream FT (140), into the well, CA (150) can be calculated as CA=CA0*FA/(FA+FT).
FIG. 2 further describes potential sources of error in the concentration of chemical as delivered for the treatment as a result of using the control strategy described above. A “bucket check” shown as point (210) and error (220) resulted in a deviation (230) of the assigned set point to the flow FASP (240) from the desired set point FASPreal (250). In addition, the conditions during the trip caused the delivered flow rate FA (260) to drift from the original set point by an additional error (270). From the calculation of the concentration delivered into the well CA in FIG. 1, it is clear that any error in delivering the exact flow rate FA would subsequently result in an error on the delivered concentration. From these examples, it is demonstrated that open loop control strategies can be a source of error when attempting to deliver accurate concentrations of chemicals during well service treatments.
FIG. 3 shows a modified state with respect to control of chemical composition in well service treatments when a closed loop strategy is used as a method for fluid delivery control. In the figure, a chemical A (310), sourced from a chemical manufacturer in a fluid form, such as to deliver a concentration of chemical per unit volume CA0, is pumped through a metering pump (320) at a flow rate FA. The flow rate is typically set by a frequency, voltage, or current proportional to the pumping rate of the pump, as typically determined by calibration, and in multiple occasions might be verified by a mass flowmeter (330). Frequently the calibration is set by the pump manufacturer, given a certain set of physical parameters in the pump (range, size of stroke, etc), and verified prior to the job execution by a volumetric determination of fluid delivered in a pre-set amount of time, what is commonly called a “bucket check.” In addition, and electronic feed-back control loop (340) is established comparing the required set point as per the bucket calibration FASP (350) to the actual measurement as determined by the flow meter FA (360), and modifying the input signal to the pump w (370) according to known control algorithm, typically a PID (proportional Integral Derivative) controller based on the measured difference between both set point and actual value. The actual concentration of chemical A delivered through stream FT (380), into the well CA (390) can be calculated as CA=CA0*FA/(FA+FT). Typically PID controllers are very effective to maintain the desired set-point, and thus the actual delivered concentration can be assumed to be close to the averaged value CA=CA0*FASP/(FASP+FT).
FIG. 4 further describes a potential source of error in the concentration of chemical a delivered for the treatment as a result of using the control strategy described above for FIG. 3. A “bucket check” shown as point (410) and error (420) resulted in a deviation (430) of the assigned set point to the flow FASP (440) from the desired set point FASPreal (450). In addition, the conditions during the trip caused the delivered flow rate FA (460) to drift from the original set point by an additional error (470). This error is minimized by the feed-back loop controller as compared to that of FIG. 2, as a result of the signal delivered to the pump w (480) varying as a response to the treatment conditions causing a drift in the flow delivered by the pump at constant rate as per FIG. 2. From the calculation of the concentration delivered into the well CA in FIG. 3, it is clear that any error in delivering the exact flow rate FA would subsequently result in an offset error on the delivered concentration. From FIGS. 3 and 4 it is demonstrated that closed loop control strategies, while typically more reliable than open loop strategies at maintaining a constant output, can also be a source of error when attempting to deliver accurate concentrations of chemicals during well service treatments, due to the offset error intrinsic to the “bucket check.” Those skilled in the art will select the appropriate control strategy given the equipment available on location, and the required flow rates to be deliver. Since the equipment deployed is to be used for treatments involving high pumping rates as well as low pumping rates, it often occurs that the target flow rates are not necessarily fitting in the optimum range for delivery by the field equipment. In some cases, it is necessary to resort to dilution of the chemicals in location in order to minimize the pumping associated errors.
FIG. 5 further shows potential sources of error in the concentration of chemical A as delivered for a treatment as a result of the variability associated with manufacturing, handling and dilution of the chemical. The chemical plant manufacturing aims to obtain a constant chemical concentration CA0 (510) but in reality a distribution of chemical compositions (520) within the lower (530) and upper (540) control limits CALSL CAUSL respectively, is obtained. Additional variability is obtained when such distribution is diluted three times aiming to obtain a target concentration ⅓ CA0 (550) producing in return a new distribution of concentrations (560).
To summarize, a method for estimating and/or confirming a fracturing fluid composition as the fluid is formed, before it is introduced to the wellbore that is effective, quick, and economical is needed.